The present invention relates to filter-press electrolyzers and, more particularly, to a filter-press electrolyzer which is characterized by the arrangement of partitions that divide the electrolyte between a pair of adjacent electrode chambers.
Filter-press electrolyzers are widely used for the electrolytic production of organic substances, the electrolysis of brine, etc., including the production of chlorine and caustic soda by the electrolysis of salt.
Filter-press electrolyzers used for the electrolysis of salt, which is a typical example of electrolytic processes that use a filter-press electrolyzer, include two different types, that is, a bipolar filter-press electrolyzer and a monopolar filter-press electrolyzer. The bipolar filter-press electrolyzer is arranged as follows: A multiplicity of bipolar electrolyzer units, which are formed by electrically and mechanically connecting together a pair of anode and cathode chambers divided by a partition, are stacked with a cation-exchange membrane interposed between each pair of adjacent units. Further, an end electrode chamber unit having an anode on one side thereof is stacked on one end of the stack of the electrolyzer units, while an end electrode chamber having a cathode on one side thereof is stacked on the other end, and the resulting stack is fixed by a hydraulic press or other similar device. The monopolar filter-press electrolyzer is constructed such that a multiplicity of anode chamber units and cathode chamber units, each having the same electrode on each side of an electrode chamber frame, are stacked with a cation-exchange membrane interposed between each pair of adjacent units, and an electrode chamber unit having an anode on one side thereof is stacked on one end of the stack of the units, while an electrode chamber unit having a cathode on one side thereof is stacked on the other end of the stack. The electrode chamber units in the monopolar filter-press electrolyzer are each provided with downcomers, ribs, etc. for reinforcing the electrode chamber frame and also for promoting the circulation of the electrolyte, and the electrodes are attached to the ribs or the like. Usually, these electrode chamber units have no partition for dividing the electrolyte.
On the other hand, the electrode chamber units of the bipolar filter-press electrolyzer are provided with partitions for dividing the anode and cathode chambers and also for transmitting the electrolytic current. Diaphragms that divide a pair of anode and cathode chambers are provided with an anode and a cathode, respectively. Either of the anode and cathode chambers is placed in an acidic environment, and the other in a reducing environment, depending upon the desired electrolytic reaction. Particularly, in the electrolysis of salt, which is a typical electrolytic process that uses an ion-exchange membrane, chlorine is formed at the anode, while highly concentrated sodium hydroxide and hydrogen are formed at the cathode. The anode chamber is formed of a thin-film forming metal, e.g., titanium, tantalum, zirconium, etc., which has high resistance to corrosion from chlorine or the like, or an alloy of such a metal. Under the atmosphere in the cathode chamber, titanium absorbs hydrogen and becomes brittle. Therefore, titanium, which has high resistance to corrosion, cannot be used for the cathode chamber.
For this reason, a ferrous metal or alloy, e.g., iron, nickel, stainless steel, etc., is used for the cathode chamber. Electrical joint can be formed by defining each electrode chamber by a partition of a metallic material and joining the partitions together. However, if titanium that constitutes the anode chamber is welded directly to a ferrous metal, e.g., iron, nickel, stainless steel, etc., which constitutes the cathode chamber, the titanium and the ferrous metal form an intermetallic compound. Therefore, it is impossible to obtain a joint structure having practical strength.
Under these circumstances, various proposals have been made for monopolar electrolyzers. For example, Japanese Patent Application Post-Exam Publication No. 53-5880 (1978) discloses a technique wherein a member of the anode chamber and a member of the cathode chamber are joined together by using a bolt that extends through a partition made of a synthetic resin material.
Japanese Patent Application Post-Exam Publication No. 52-32866 (1977) discloses a technique wherein a partition is formed from a plate-shaped member made of a ferrous metal and titanium which are joined by explosive welding, and ribs are welded to both surfaces of the partition, and then an anode and a cathode are welded to the ribs. Japanese Patent Application Post-Exam Publication No. 56-36231 (1981) uses a composite material formed by joining together titanium and iron with copper sandwiched therebetween. The titanium of the composite material is welded to titanium that constitutes an anode-side partition of a bipolar electrolyzer unit, and the iron of the composite material is similarly welded to a cathode-side partition made of a ferrous metal.
As has been described above, there are various types of partition used in bipolar electrolyzers. In any type of electrolyzer, ribs are connected to a partition, and an electrode is attached to the ribs by welding or other similar method. With this arrangement, however, a voltage drop due to the ribs is unavoidable. In addition, it is necessary to use a special method for joining together the cathode-side metal and the anode-side metal.
To solve these problems, a bipolar electrolyzer has been proposed as Japanese Patent Application Laid-Open (KOKAI) No. 03-249189 (1991) [Japanese Patent Application No. 02-45855 (1990)], which includes an electrolyzer unit having a partition plate formed from two plates pressed to have recesses and projections, which fit to each other, and electrodes are joined to the projections on both sides of the partition plate, thereby providing a simplified structure and facilitating the process for producing the electrolyzer.
In an electrolytic reaction that generates a large amount of gas as in the electrolysis of salt by the ion-exchange membrane method, a region where the content of gas generated or the content of bubbles in the electrolyte is relatively high is formed in the upper part of the electrode chamber. It is known that a region where a gas or bubbles reside has an adverse effect on the ion-exchange membrane in long runs. To reduce the area where a gas or bubbles reside, various schemes have heretofore been carried out: For example, a scheme of optimizing the position of installation of a nozzle for allowing the electrolyte or the gas generated to flow out to the outside; and a scheme of preventing bubbles from contacting the ion-exchange membrane by providing a gas-liquid separating chamber in the upper part of the electrolyzer unit. In an electrolyzer having a large electrode area, if the current distribution in an electrode chamber becomes nonuniform, a phenomenon that is unfavorable for the electrolyzer performance occurs, for example, local wear of the electrodes, and local deterioration of the ion-exchange membrane. Therefore, consideration is given to the position of installation of the electrodes and current collecting members so that the path of current, i.e., anode--partition--cathode--anode, is substantially uniform, thereby allowing the current distribution in the electrode chamber to become uniform.
In addition, it has been schemed to minimize the electrolyte concentration and temperature distributions in the electrode chamber. To minimize these distributions, the conventional practice is to increase the speed or rate of circulation of the electrolyte that is externally supplied into the electrode chamber and discharged therefrom. However, a large-sized circulating device is needed in order to increase the rate of circulation, and satisfactory effect cannot necessarily be obtained in terms of the achievement of a uniform concentration or temperature of the electrolyte.
In the case of an electrolyzer unit formed by pressing flat plates, a region where a gas resides unavoidably occurs in the upper part of the electrode chamber even if consideration is given to the position of installation of an outlet nozzle for the electrolyte or the gas generated.
An effective way of making the electrolyte concentration or temperature uniform is to allow the electrolyte to be uniformly supplied to the electrode chamber. In an electrolyzer unit formed by pressing flat plates, however, an electrolyzer frame member is provided in the lower part of the electrolyzer unit, and it is therefore impossible to provide a device for dispersing the electrolyte. Similarly, it is impossible to provide a gas-liquid separating device for the electrolyte in the upper part of the electrolyzer unit.
The present inventors have previously proposed an electrolyzer unit formed by pressing flat plates and also proposed an electrolyzer wherein an electrolyte dispersing and feeding chamber is provided in the lower part of an electrolyzer unit, and a gas-liquid separating chamber is provided in the upper part of the unit, in Japanese Patent Application Nos. 03-154687 (1991), 03-154688 (1991) and 03-160260 (1991) (U.S. patent application Ser. No. 07/904251), etc.
With the proposed techniques, the quantities of the electrolyte and the generated gas immediately before they are discharged from the electrode chamber to the gas-liquid separating chamber provided in the upper part of the chamber are uniformly distributed in the horizontal direction of the electrolyzer. However, in the gas-liquid separating chamber, the flow rate of the fluid comprised of a gas, a gas-liquid multi-phase flow, a liquid, etc. increases as the fluid approaches the discharge opening. In addition, the speed of the fluid in the chamber increases, and the pressure loss also increases.
Consequently, a pressure difference is produced inside the gas-liquid separating chamber between the discharge side and the side opposite to it. As a result, the gas-liquid multi-phase flow pulsates, causing the pressure in the electrolytic chamber to fluctuate. The fluctuation of pressure in the electrolytic chamber causes vibration of the ion-exchange membrane that divides the anode and cathode chambers, giving rise to problems such as damage to the ion-exchange membrane.
An object of the present invention is to provide an electrolyzer having a gas-liquid separating chamber provided in the upper part of an electrolyzer unit formed by pressing flat plates. The electrolyzer is arranged to prevent vibration of the ion-exchange membrane due to the fluctuation of pressure in the electrolytic chamber caused by pulsation of the gas-liquid multi-phase flow or the like which occurs inside the gas-liquid separating chamber, thereby stabilizing the operation of the electrolyzer and also enabling the ion-exchange membrane to be stably used for a long period of time.